The present invention relates to structures for attaching together or sealing a space between components having different coefficients, rates, or amounts of thermal expansion. In particular, the present invention relates to structures for attaching high-temperature shielding components of gas turbine engines to adjacent components thereof or for sealing a space between such components.
It has long been a problem to support components that are subject to thermal expansion and contraction with support members that are also subject to such thermal expansion and contraction but at a different rate. If the components and support member are rigidly connected, each will be subject to stresses that may eventually lead to their mechanical failure. This problem is particularly acute in the case in which either or both are made from a brittle material such as a ceramic. This problem, for example, can arise in gas turbine engines and, in particular, in the combustion and turbine regions of such engines.
Gas turbine engines, such as those providing thrust power to aircraft or those generating electrical power, conventionally comprise a casing enclosing alternate, stages of rotary and stationary aerofoil blades positioned in an annular gas passage. In order to ensure an efficient operation of such turbine engines, it is important that the clearances between the tips of the rotary aerofoil blades and the radially outer wall of the gas passage are as small as possible. If the clearances are too great, excessive gas leakage occurs across the blade tips, thereby reducing turbine efficiency. There is a danger, however, that if clearances are reduced so as to reduce leakage, it is likely that under certain turbine operating conditions, the tips of the rotary blades will make contact with the gas passage wall, thereby causing both blade and wall damage.
In an attempt to ensure that optimum blade tip clearances are achieved and maintained with minimal gas leakage across them, it has been conventional to surround a stage of rotary aerofoil blades with a shroud ring. The shroud ring is conventionally attached to the turbine casing in such a manner that it provides a radially inner surface that defines a portion of the radially outer wall of the turbine annular gas passage. Although shroud rings are quite easily made with high tolerances, challenges still exist in ensuring that optimum tip clearances are maintained during turbine operation. These challenges are associated mainly with the different rates of the thermal expansion of the turbine casing, the shroud ring, and the aerofoil blade assembly. Thus, for example, even if the turbine casing and the: shroud ring have similar thermal expansion coefficients, the differences in their masses and in the temperatures to which they are exposed during turbine operation still result in different rates or amounts of expansion or contraction. Consequently, there is a danger of the shroud ring and possibly the turbine casing being distorted.
Similarly, the combustion chamber of a gas turbine engine can be lined with a ceramic combustion chamber liner (or also commonly known as combustor liner) to protect the combustion chamber casing, which is made of a metal, from the very high temperature of the combustion gas. The different coefficients or rates of thermal expansion or contraction of the ceramic and metal also present a danger that either or both would be distorted and mechanical stresses would develop as a result.
Therefore, it is very desirable to provide structures for attaching together or supporting components having different coefficients, rates, or amounts of thermal expansion or contraction so to mitigate or eliminate the danger of shape distortion of these components that could lead to their premature failure. It is also very desirable to provide structures for supporting liners of high-temperature sections of gas turbine engines, which structures help to maintain optimum clearances between the liners and the adjacent elements of the turbine. Furthermore, it is also very desirable to provide structures to seal the space between two components having different coefficients, rates, or amounts of thermal expansion or contraction so to substantially prevent a communication between a medium present in the space and another medium present in the internal space of one component.